#sea urchin eggs

Regeneration -ll

Sea urchin eggs activate upon fertilization primarily due to:

Egg activation involves resumption of meiosis, elevation of metabolism, and preparation for embryonic cleavage. Central event is single propagated calcium wave sweeping from sperm entry point throughout cytoplasm, raising cytosolic calcium over tenfold. Calcium activates calmodulin-dependent kinase II leading to cyclin B degradation and release from M-phase arrest, stimulates NAD kinase increasing NADPH, triggers cortical granule fusion, and opens Na+/H+ exchangers raising pH to promote protein synthesis. ATP increase, pH rise, and sperm nuclear incorporation are downstream consequences of calcium signaling. Without calcium transient, eggs remain arrested even if sperm fuses, proving calcium rise as primary activator.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Calcium wave as primary activator of sea urchin egg metabolism.

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Regeneration -ll

The fast block to polyspermy lasts approximately:

Fast block is transient electrical depolarization preventing extra sperm fusion until slow block completes. Sea urchin resting potential -70 mV shifts to +20 mV within seconds after first sperm entry. This positive potential makes membrane refractory because sperm-egg fusion voltage-dependent and favored only at negative potentials. Depolarization lasts about a minute until cortical granule exocytosis elevates fertilization envelope. Voltage clamp experiments show positive-held eggs reject sperm, return to negative restores receptivity. One to two minutes ensures overlap with slow block; shorter seconds insufficient, longer many minutes would impair subsequent ionic homeostasis required for development.

Ref: Jaffe LA, Fast block to polyspermy in sea urchin eggs - membrane potential duration about one minute, CSIRO Reproduction.

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Regeneration -ll

Sea urchin eggs attract sperm via:

Sea urchin reproduction involves broadcast spawning where eggs and sperm meet in seawater at low concentrations, requiring efficient guidance mechanisms. Eggs achieve this via diffusible small peptides that establish steep chemical gradients attracting sperm by chemotaxis. Peptides bind flagellar receptors triggering guanylate cyclase, cGMP rise, potassium efflux, intracellular alkalization, and CatSper-mediated calcium pulses causing transient flagellar asymmetry and turning up-gradient. Repeated sampling creates biased random walk toward egg source. Electrical signals, temperature gradients, or phototaxis play negligible roles in this marine invertebrate, where chemotaxis optimizes fertilization efficiency and species-specificity via peptide-receptor matching.

Ref: Nature Review, Kaupp et al., Chemotaxis in sea urchin sperm - resact-guided navigation via Ca2+ signaling.

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Sea urchin -l

Sea urchin eggs attract sperm via:

Broadcast spawning sea urchins release gametes freely into seawater where sperm must locate relatively immobile eggs efficiently to ensure reproductive success. Egg jelly coat surrounding oocyte releases species-specific peptides that diffuse forming stable chemical gradient extending millimeters. Spermatozoa sense gradient concentration via receptor guanylate cyclase and CatSper Ca2+ channels, adjusting flagellar waveform asymmetry to bias swimming up gradient, classic chemotaxis analogous to bacterial navigation. Electrical signals, temperature gradients or light cues are not directional guides for marine gametes. This peptide-based chemoattraction substantially increases fertilization probability. Resact and speract peptides exemplify chemical attractants guiding sperm propulsion in open ocean.

Ref: Kaupp et al., Annu Rev Physiol, Chemotaxis in sea urchin sperm; Gilbert Chapter 7: Egg jelly chemotaxis.

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